Atlantic meridional overturning circulation

The Atlantic Meridional Overturning Circulation (AMOC) is a major current in the Atlantic Ocean, characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder water in the deep Atlantic. The AMOC is an important component of the Earth’s climate system.

This ocean current system transports a substantial amount of heat energy from the tropics and Southern Hemisphere toward the North Atlantic, where the heat is then transferred to the atmosphere. Changes in this ocean circulation could have a profound impact on many aspects of the global climate system.

There is growing evidence that fluctuations in Atlantic sea surface temperatures, hypothesized to be related to fluctuations in the AMOC, have played a prominent role in significant climate fluctuations around the globe on a variety of time scales.

Measurements across the North Atlantic suggest multidecadal swings in sea surface temperatures that may be at least in part due to fluctuations in the AMOC. The figure below describes this variation in North Atlantic sea surface temperatures for the period 1856 to 2009. The repeatitive cycle obvious in this figure is known as the Atlantic Multidecadal Oscillation (AMO). Evidence from paleorecords suggests that there have also been large, decadal-scale changes in the AMOC, particularly during glacial times. These abrupt changes have had a profound impact on climate, both locally in the Atlantic and in remote locations around the globe.

At its northern boundary, the AMOC interacts with the circulation of the Arctic Ocean. The summer Arctic sea ice cover has undergone dramatic retreat since satellite records began in 1979, amounting to a loss of almost 30% of the September ice cover in 29 years. Climate model simulations suggest that rapid and sustained September Arctic ice loss is likely in future 21st century climate projections.

Monthly May ice extent for 1979 to 2011 shows a decline of 2.4% per decade. Source: National Snow and Ice Data Center.

Because the AMOC's heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change, there have been questions about the likelihood of an "collapse" or an abrupt change. In the a 2008 study on Abrupt Climate Change by the U.S. Climate Change Science Program, the following conclusions were drawn :

It is very likely that the strength of the AMOC will decrease over the course of the 21st century in response to increasing greenhouse gases, with a best estimate decrease of 25–30%.

Even with the projected moderate AMOC weakening, it is still very likely that on multidecadal to century time scales a warming trend will occur over most of the European region downstream of the North Atlantic Current in response to increasing greenhouse gases, as well as over North America.

It is very unlikely that the AMOC will undergo a collapse or an abrupt transition to a weakened state during the 21st century.

It is also unlikely that the AMOC will collapse beyond the end of the 21st century because of global warming, although the possibility cannot be entirely excluded.

Although it is very unlikely that the AMOC will collapse in the 21st century, the potential consequences of this event could be severe. These might include a southward shift of the tropical rainfall belts, additional sea level rise around the North Atlantic, and disruptions to marine ecosystems.

Background

The oceans play a crucial role in the climate system. Ocean currents move substantial amounts of heat, most prominently from lower latitudes, where heat is absorbed by the upper ocean, to higher latitudes, where heat is released to the atmosphere. This poleward transport of heat is a fundamental driver of the climate system and has crucial impacts on the distribution of climate as we know it today. Variations in the poleward transport of heat by the oceans have the potential to make significant changes in the climate system on a variety of space and time scales.

In addition to transporting heat, the oceans have the capacity to store vast amounts of heat. On the seasonal time scale this heat storage and release has an obvious climatic impact, delaying peak seasonal warmth over some continental regions by a month after the summer solstice. On longer time scales, the ocean absorbs and stores most of the extra heating that comes from increasing greenhouse gases (Levitus et al., 2001), thereby delaying the full warming of the atmosphere that will occur in response to increasing greenhouse gases.

One of the most prominent ocean circulation systems is the Atlantic Meridional Overturning Circulation (AMOC). As described in subsequent sections, and as illustrated below, this circulation system is characterized by northward flowing warm, saline water in the upper layers of the Atlantic (red curve), a cooling and freshening of the water at higher northern latitudes of the Atlantic in the Nordic and Labrador Seas, and southward flowing colder water at depth (light blue curve). This circulation transports heat from the South Atlantic and tropical North Atlantic to the subpolar and polar North Atlantic, where that heat is released to the atmosphere with substantial impacts on climate over large regions.

The Atlantic branch of this global MOC consists of two primary overturning cells:

an “upper” cell in which warm upper ocean waters flow northward in the upper 1,000 meters (m) to supply the formation of North Atlantic Deep Water (NADW), which returns southward at depths of approximately 1,500-4,500 m; and,

a “deep” cell in which Antarctic Bottom Waters (ABW) flow northward below depths of about 4,500 m and gradually rise into the lower part of the southward-flowing NADW.

Of these two cells, the upper cell is by far the stronger and is the most important to the meridional transport of heat in the Atlantic, owing to the large temperature difference (~15° C) between the northward-flowing upper ocean waters and the southward-flowing NADW.

Schematic of the ocean circulation (from Kuhlbrodt et al., 2007) associated with the global Meridional Overturning Circulation (MOC), with special focus on the Atlantic section of the flow (AMOC). The red curves in the Atlantic indicate the northward flow of water in the upper layers. The filled orange circles in the Nordic and Labrador Seas indicate regions where near-surface water cools and becomes denser, causing the water to sink to deeper layers of the Atlantic. This process is referred to as “water mass transformation,” or “deep water formation.” In this process heat is released to the atmosphere. The light blue curve denotes the southward flow of cold water at depth. At the southern end of the Atlantic, the AMOC connects with the Antarctic Circumpolar Current (ACC). Deep water formation sites in the high latitudes of the Southern Ocean are also indicated with filled orange circles. These contribute to the production of Antarctic Bottom Water (AABW), which flows northward near the bottom of the Atlantic (indicated by dark blue lines in the Atlantic). The circles with interior dots indicate regions where water upwells from deeper layers to the upper ocean (see Section 2 for more discussion on where upwelling occurs as part of the global MOC).

In assessing the “state of the AMOC,” we must be clear to define what this means and how it relates to other common terminology. The terms Atlantic Meridional Overturning Circulation (AMOC) and Thermohaline Circulation (THC) are often used interchangeably but have distinctly different meanings. The AMOC is defined as the total (basin-wide) circulation in the latitude depth plane, as typically quantified by a meridional transport streamfunction. Thus, at any given latitude, the maximum value of this streamfunction, and the depth at which this occurs, specifies the total amount of water moving meridionally above this depth (and below it, in the reverse direction). The AMOC, by itself, does not include any information on what drives the circulation.

In contrast, the term “THC” implies a specific driving mechanism related to creation and destruction of buoyancy. Rahmstorf (2002) defines this as “currents driven by fluxes of heat and fresh water across the sea surface and subsequent interior mixing of heat and salt.” The total AMOC at any specific location may include contributions from the THC, as well as contributions from wind-driven overturning cells.

It is difficult to cleanly separate overturning circulations into a “wind-driven” and “buoyancy-driven” contribution. Therefore, nearly all modern investigations of the overturning circulation have focused on the strictly quantifiable definition of the AMOC as given above. We will follow the same approach in this report, while recognizing that changes in the thermohaline forcing of the AMOC, and particularly those taking place in the high latitudes of the North Atlantic, are ultimately most relevant to the issue of abrupt climate change.

There is growing evidence that fluctuations in Atlantic sea surface temperatures (SSTs), hypothesized to be related to fluctuations in the AMOC, have played a prominent role in significant climate fluctuations around the globe on a variety of time scales. Evidence from the instrumental record (based on the last ~130 years) shows pronounced, multidecadal swings in SST averaged over the North Atlantic.

These multidecadal fluctuations may be at least partly a consequence of fluctuations in the AMOC. Recent modeling and observational analyses have shown that these multidecadal shifts in Atlantic temperature exert a substantial influence on the climate system ranging from modulating African and Indian monsoonal rainfall to influencing tropical Atlantic atmospheric circulation conditions relevant to hurricanes. Atlantic SSTs also influence summer climate conditions over North America and Western Europe.

Evidence from paleorecords (discussed more completely in subsequent sections) suggests that there have been large, decadal-scale changes in the AMOC, particularly during glacial times. These abrupt change events have had a profound impact on climate, both locally in the Atlantic and in remote locations around the globe. Research suggests that these abrupt events were related to massive discharges of freshwater into the North Atlantic from collapsing land-based ice sheets. Temperature changes of more than 10o C on time scales of a decade or two have been attributed to these abrupt change events.